A polarizing element is used in a liquid crystal display. The polarizing element generally comprises a polarizing membrane and a transparent protective film.
The polarizing membrane usually comprises a polymer film containing iodine or a dichromatic dye. The iodine or the dichromatic dye is incorporated in or adsorbed on a polymer film. The film is then subjected to a uniaxial stretching. Polyvinyl alcohol is generally used as the polymer. The dichromatic dye can be adsorbed on a polymer film that is previously subjected to a uniaxial stretching to prepare a polarizing membrane.
The transparent protective film generally is a cellulose triacetate film. The polarizing membrane is usually provided between two transparent protective films.
A conventional polarizing element comprises a polarizing membrane formed by stretching a polymer film. Therefore, the direction of orientation is restricted in a certain direction.
Japanese Patent Provisional Publication No. 7(1995)-261024 discloses a method for preparing a polarizing plate. In the method, a layer containing optical active molecules and another layer containing a dichromatic dye are formed on a substrate, and then exposed to light so that the dichromatic dye molecules are aligned in a definite direction. According to this method, a polarizing plate having a polarizing axis in the definite direction can be produced without stretching. However, it takes such long time to align the dye molecules by light-exposure that a long band-shaped plate cannot be continuously treated by this method. Further, the alignment oriented in the plane by light-exposure is poor in evenness. Furthermore, a polarization degree (which is the most important matter as a polarizing plate) of the resultant plate is relatively poor, as compared with a conventional polarizing plate.
J. F. Dreyer (J. Phys. Colloid Chem., 808(1948), pp. 52) proposes a method in which a glass plate or a polymer film is beforehand subjected to rubbing treatment (beforehand rubbed with cloth or paper in a definite direction) and onto the treated surface a dichromatic dye is adsorbed. However, thus obtained orientation is liable to relax under the circumstances of high temperature and humidity, and accordingly the alignment of dichromatic dye molecules is often disturbed to lower the polarization degree.
Japanese Patent Provisional Publication Nos. 3(1991)-54506 and 3(1991)-58004 disclose another method for preparing a polarizing plate. In the method, a film of cellulose triacetate or polyethylene terephthalate is subjected to the rubbing treatment, and then the dye is added. However, even the thus produced polarizing plate exhibits a low polarization degree.
U.S. Pat. Nos. 2,400,877 and 2,544,659 disclose a process for preparation of a polarizing element. The process comprises the steps of: applying a solution of dichromatic dye onto a substrate, evaporating the solvent while molecules of the dye are aligned in a nematic phase, and fixing the alignment slowly. The resultant polarizing element, however, exhibits a low polarization degree, and is poor in heat-resistance.